I googled the answer, here are the instructions:
Open your Cemu folder and find the "graphicPacks" folder, inside you should see a bunch of folders with game titles and their resolutions, find the folder of the game you want to play, I will be using MarioKart 8 as an example. In my case the folder would be titled "MK8_1080p - Copy" so I would rename it to "MK8_720p" if I wanted a 1280x720 resolution. After this you will have to enter the folder and open the text file called "rules", here you must change the "overwriteHeight" & "overwriteWidth" values to the resolution that corresponds with the folder name. From here you can save the file and open Cemu, once opened tap the Options dropdown menu and click Graphic packs, now you can tick the graphic pack that you just created and open your WUD.
This info was literally all from google (re-worded by me) so don't give me credit but I hope this helps.